1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine rotor blade tip cap.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine includes a turbine with multiple rows or stages of rotor blades and stator vanes to guide a hot gas flow through and extract mechanical energy to drive the compressor or even an electric generator in the case of an industrial gas turbine (IGT) engine. The efficiency of the engine can be increased by passing a higher gas flow temperature through the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine vanes and blades and to the amount of cooling provided to these airfoils.
Another problem with turbine a turbine, especially the IGT engine, is that the rotor blades have tips that form a seal with an outer shroud of the turbine casing to create a blade outer air seal (or, BOAS) to limit hot gas flow leakage across the blade tip gaps. The blade tip gap can vary in size due to engine thermal loads and engine transient conditions. Blade tips can also rub against the shroud surface such that the gap is zero. Blade rub can sometimes be beneficial when proper design for blade tip rub is accounted for. However, hot gas flow leakage across the blade tip gap will result in not only lower turbine efficiency but also hot spots on the tips that result in erosion of tip material that limits the blade life. An IGT engine can operate continuously for over 40,000 hours so blade tip erosion can cause serious problems. An engine shutdown could even be required which will cause further problems with the engine operator. Thus, blade tip cooling is an important design objective for a rotor blade in a gas turbine engine, especially for an IGT engine.
When a blade tip of a turbine rotor blade is damaged, the entire blade must be replaced. A typical rotor blade can have a squealer pocket formed on the tip in which tip rails extend around the periphery of the blade tip along the pressure side wall and the suction side wall and joined around the leading edge wall. The tip rails provide for a small surface area for tip rub as well as a seal against the outer shroud surface.